Alkaline membrane fuel cells in principle can operate without added liquid electrolyte(s) and thereby rely fully on the ionic conductivity of well-hydrated, anion conducting membranes configured to conduct hydroxide ions (OH−). A liquid electrolyte-free mode of operation has strong advantages in comparison to liquid electrolyte-based fuel cells, as the wide selection of PEM fuel cell technologies employing proton conducting membrane electrolytes shows. Compared to the more established and developed PEM fuel cell technologies, the alkaline membrane fuel cell (AMFC) has great advantages with respect to using catalysts other than expensive platinum group metals and employing inexpensive metal hardware. An exemplary AMFC is described in the above mentioned U.S. patent application Ser. No. 12/477,669, filed Jun. 3, 2009, and assigned to the same assignee of the present invention. On the other hand, the AMFC poses a significant challenge derived from its use of OH− ion conducting polymers that demonstrate a more limited ionic conductivity. In addition, the high sensitivity of this ion conductivity to the water levels of an operating AMFC poses an additional challenge. Ionic conductivity challenges manifest themselves particularly in the composition and the mode of fabrication of the two catalyst layers that bond to the major surfaces of the membrane electrolyte to form the catalyst-coated membrane (CCM). These composition and fabrication challenges are more severe with an AMFC design because the effective specific ionic conductivity within the catalyst layer is, typically, an order of magnitude lower than that of the membrane electrolyte. This is because the ionic material of the catalyst layer fills only a fraction of its total volume. In addition, the precise details of the morphology of the ionic network that results from mixing the catalyst and the ionomer material determines whether the ionic network enables effective ionic access through the thickness of the catalyst layer, as is required for high catalyst utilization.
FIG. 1 includes a schematic illustration of a prior art catalyst layer formed from mixtures of an ion-conducting polymer/ionomer 10 (shaded haze) and metal catalyst particles 12 (spheres) disposed in a catalyst layer between a porous carbon gas diffusion layer (GDL) 14 and a cell membrane (membrane) 16. One might expect that in a desirable catalyst microstructure the cohabitating networks of metal catalyst 12 and ionomer 10, as shown in FIG. 1, each provide good connectivity through the thickness dimension of the catalyst layer, as well as good ionic access to a large fraction of the catalyst sites. As depicted in FIG. 1, the ionomer volume fraction is relatively high. One therefore might expect, at least in principle, an effective ionic conductivity in this catalyst layer. This catalyst layer reflects the state-of-the-art approach of enhancing ionic conductivity in an AMFC catalyst based on use of a high fraction of recast ionomer in the catalyst mix. However, the catalyst structure of FIG. 1 likely has significant drawbacks. For instance, the high volume fraction of the ionic material 10 can tend to isolate the metal catalyst particles 12 from each other, as indicted by arrows 20 and 22 in FIG. 1, such that electronic contact of the catalyst particles to the electron current collector along the side of the gas diffusion layer 14 is compromised. As a result, these catalyst particles 12 can be totally inactive.
Furthermore, a crust 24 of excess recast polymer electrolyte 10 will tend to form on both major surfaces of the catalyst layer, as shown in FIG. 1. These crusts 24 can thereby define the nature of interfaces of the catalyst layer with the membrane and with the gas diffusion layer 14. A potential problem with such interfacial crust 24 is that the state of the water level or hydration within an active part of the catalyst layer may be determined by the water transport and water capacity characteristics of these interfacial crusts 24. For example, dry-out of an ionomer crust 24 formed between the AMFC cathode catalyst layer and the alkaline electrolyte membrane 16 could slow the rate of re-hydration of the catalyst layer because the crust 24 needs re-hydration as a prerequisite for effective re-hydration of the catalyst-containing part of the layer. An excess of ionomer at an interface of the catalyst layer with the gas diffusion layer will cause high electronic resistance between the gas diffusion layer and the metal catalyst particles, leading potentially to an electronic disconnect.
One may draw the conclusion from these observations and arguments that an optimum volume fraction of OH− ion conducting ionomer is a basis and an objective for fine tuning catalyst ink preparations and applications in order to achieve highly performing AMFC cathodes. This objective proves difficult to achieve. Relevant reports published to date disclose that the maximum power density that H2/O2 AMFCs with platinum (Pt) catalysts achieved is 100-200 mW/cm2. Importantly, once a non-precious metal catalyst replaces the Pt cathode catalyst, the maximum AMFC power density achieved to date is about 50 mW/cm2. It is understood that the relative low performance of AMFCs with Pt catalysts versus that of Pt-catalyzed PEMFCs is the result of the relatively low ionic (OH−) conductivity in the recast ionomer material used as the bonding agent and ionic conductor in the AMFC catalyst layer. The performance penalty that results from the lower conductivity of the OH− conducting ionomer is exacerbated further by the low catalyst activity of non-Pt catalysts reported to date. The low AMFC performance achieved thus far has cast serious doubts on the ability to reduce to practice the central AMFC advantage of using non-Pt catalysts.
Clearly, the critical requirement for reducing AMFC technology to practice has been the development of an optimal catalyst layer composition and structure based on a non-Pt catalyst and a recast alkaline electrolyte membrane material that will enable good ionic and electronic connectivity through the catalyst layer, as well as prevent the formation of ionomer crusts. In addition, another critical requirement includes packaging a sufficiently large surface area of the catalyst into an overall thickness of the catalyst layer that does not exceed several micrometers, such as less than 10 micrometers. Fulfilling this requirement may soften the demand for a high specific conductivity of the recast ionomer because the overall limited thickness of the catalyst layer caps the maximum length of ionic routes to the active catalyst sites.
In the more fully developed area of CCM fabrication for PEM fuel cells, the catalyst of choice has been carbon-supported Pt. In addition, the carbon support of choice has been carbon XC-72, a product available from Cabot Corporation of Billerica, Mass. An important feature of this type of high surface area carbon is its “very open” structure. Carbon powder is mixed with recast Nafion® available from DuPont of Wilmington, Del. This composite mixture is laid down as a thin layer and a void fraction of about 50% is maintained. The open structure on micron and sub-micron scales is ascribed to the XC-72 material. This high void volume of catalyst material appears to be an important enabler to achieve an effective micrometer scale “interweave” between the carbon and the recast ionomer. Moreover, this “interweave” structure appears to be a key to leaving some open space for gas penetration into the composite catalyst layer.
Recent attempts to use non-Pt catalysts in AMFC designs are based to a significant degree on similar high-surface area carbon supports with the Pt catalytic sites replaced by catalyst centers based typically on adsorbed and heat-treated cobalt complexes. In the latter case, the AMFC performance obtained was quite low due to the relatively very low packing density of the catalyst and due to the weight % of cobalt centers, which are not more than 1% of the total weight of the carbon-supported catalyst. Consequently, a thick layer of the catalyst is required to contain enough of the active catalytic centers. With the conductivity of the recast anion-exchange membrane (AEM) material limited, effective access to a large number of catalyst sites located well inside the thickness of the relatively thick catalyst layer could not be sustained.
In principle, a catalyst layer employing a dispersed and unsupported metal catalyst is a potential solution to the needs of tighter packaging of the catalyst surface area whereby the volume of the support material is eliminated and thereby the volume occupied by the electronically conducting material is filled with metal catalyst particles. The well-known drawback of this alternative is that the degree of dispersion achievable in such metal “blacks,” of unsupported metal particles, is significantly smaller versus the case of carbon-supported catalysts because of the high tendency of the unsupported metal particles to agglomerate.
The discussion above indicates that certain features of the metal catalyst that one employs in a high performing CCM of an AMFC design are important and are required in addition to the conductivity and stability requirements of the recast ionomer used in the catalyst layer and the demands of an effective mode of CCM fabrication. Hence, a need exists in the art for AMFC designs and methods that provide non-Pt metal catalysts with certain features and structures and that meet certain requirements needed to achieve AMFC power densities of the same order as power densities generated by Pt catalyst based PEM fuel cells. In addition to the special morphological features sought for the catalyst, a specific metal for preparation of the cathode catalyst for AMFCs was sought. This search is directed by the need for an optimized surface chemical activity versus oxygen atoms and the stability of the metal in the alkaline membrane medium.
One other critical factor in the successful application of catalyst layers to membranes of the type used in AMFCs is the quality of the interlayer bond at the catalyst layer-membrane interface. A catalyst-coated-membrane (CCM) must withstand many hours of cell operation, involving strongly varying degrees of hydration. The repetitive change in ionomer state of hydration causes repetitive polymer dimensional changes and, consequently, will cause delamination along the catalyst/membrane interface unless the interfacial bond is very robust. Typical AMFC membranes and ionomers have poly(arylene) backbones and, consequently, hardly flow at the highest temperatures allowed by polymer chemo-thermal stability limits. This is to be contrasted with the per-fluorocarbon backbone of mainstream proton conducting membranes. In the latter, hot pressing the catalyst layer to the membrane does result in a robust bond, thanks to the significant thermoplasticity of the ionomeric material in the catalyst layer and in the membrane that is achievable under the chemo-thermal stability limit. This difference in thermo-plasticity makes it impossible to copy directly the catalyst layer/membrane lamination process from the mainstream PEM fuel cell technology. Alternative modes of catalyst layer/membrane interfacial bonding, therefore, must be devised for AMFC CCMs and once ionomer-ionomer interdiffusion is ruled out. A remaining option is embedding solid catalyst particles into the surface of the membrane to generate anchoring sites. The latter process can be facilitated by solvents that open up the nano-pores in the membrane surface by controlled swelling of the outer membrane surface. We have concluded that, for the latter mode of interfacial bonding, the ionomer level in the catalyst ink cannot be too high, because the highest probability of penetration of a solid catalyst particle into the membrane surface is achieved when “neat” catalyst powder is pressed onto the membrane surface, with no intervention by some added ionomeric “binder”. The binder may be required, however, for structural stabilization in the “bulk” of the catalyst layer, as well as for the benefit of ionic conductivity in the catalyst layer. Consequently, we concluded that, when the bond is to be based on membrane-embedded metal particles serving as anchors for the catalyst layer, the ionomer content in the ink has to be significantly lower than the volume ratio of ionomer:metal, 1:1, typically employed in PEM fuel cell catalyst inks, but it should not go down all the way to zero.
Finally, fuel cell cathode catalyst layers based on unsupported metal particles, have an intrinsic stability advantage vs. catalyst layers based on carbon-supported metal particles. The carbon support in a cathode catalyst is vulnerable to oxidative loss, particularly near open circuit conditions and under higher cell temperatures. The carbon support is more readily attacked than the metal; hence, elimination of the support to fully rely on a metal “black” as cathode catalyst has a important advantage in cell longevity.